In 3GPP (3rd Generation Partnership Project) Rel.8/9/10 system, Channel Quality Indicator (CQI), one type of channel state information (CSI), is an important communication parameter used for scheduling and link adaptation at eNodeB (eNB). In practice, since UE (user equipment) knows better about the downlink channel based on certain reference signal, like CRS (common reference signal) or CSI-RS (channel state information-reference signal), CQI is calculated and recommended at UE side. Then, CQI is feedback by the UE with a certain index in the CQI table, which includes some combinations of modulation order and coding rate, referring to Table 7.2.3-1 in 3GPP TS 36.213 V10.5.0, which is entirely incorporated hereto by reference.
FIG. 1 shows the CQI table in TS 36.213. It can be seen from FIG. 1 that the standard CQI table, which may also be referred to as a legacy CQI table, includes 16 entries with indices from 0-15, corresponding to modulation schemes such as QPSK, 16QAM and 64QAM. Therefore, 4 bits are necessary to reflect a certain entry when the UE feedbacks a certain wideband CQI to the eNB.
As to other CQI types defined according to different feedback modes or transmission modes, such as subband CQI, spatial CQI or UE selected CQI, UE do not directly feedback the entry/index in the same CQI table as shown in FIG. 1. Instead, an implicit mechanism is used to feedback their CQI offset level from the wideband CQI value. For example,Subband differential CQI offset level=subband CQI index−wideband CQI index.
FIG. 2 shows the subband differential CQI table in TS 36.213. It can be seen that the standard subband differential CQI table, which may also be referred to as a legacy subband differential CQI table, includes 4 entries with indices from 0 to 3. Therefore, UE needs 2 additional bits to feedback the subband differential CQI offset level.
Also, Modulation Coding Scheme (MCS) is an important communication parameter in 3GPP Rel.8/9/10 system. MCS refers to which combination of modulation order and coding rate is used in physical transmission of downlink and uplink. There is also a table, called MCS table, restricts which combination of modulation order and transport block size could be used.
FIG. 3 shows the MCS table in TS 36.213. It can be seen that the standard MCS table, which may also be referred to as a legacy MCS table, includes 32 entries with indices from 0-31, corresponding to modulation orders such as 2, 4 and 6. The last three entries with indices 29-31 are used for re-transmission. In 3GPP, which MCS is used is informed in Downlink Control Information (DCI). And 5 bits are necessary for this indication.